Fixed point math

**Legolas** · 31-05-2007, 12:11 PM

System: Windows XP sp2
Compiler: arm-nds-fpc 2.1.4. i386-win32
API: none

I need to do some conversion from floating point values to 1.19.12 fixed point. The code I have found in C is:

Code:

#define floattof32&#40;n&#41;        &#40;&#40;int32&#41;&#40;&#40;n&#41; * &#40;1 << 12&#41;&#41;&#41;

that I have tried to convert in this way:

[pascal]function floattof32(n: cfloat): cint32; inline;
begin
floattof32 := (cint32(n) * (1 shl 12));
end;[/pascal]

but does not work as I expected. Can anyone help me here?

**Robert Kosek** · 31-05-2007, 12:58 PM

Heh, I had a whole reply typed up when I noticed something. Look at the C macro again for a second. The typecast is on the result of the shift and multiplication, and not the number before the multiplication. So your function should be:[pascal]function floattof32(n: cfloat): cint32; inline;
begin
floattof32 := cint32((n) * (1 shl 12));
end;[/pascal]

**Legolas** · 31-05-2007, 01:10 PM

Uh-oh... You are right...

ops:
However it does not work too

**Robert Kosek** · 31-05-2007, 01:32 PM

Don't worry, I shot myself in the foot with a sizable if-then-else block the other day (just about driving myself nuts in the process). What exactly are you trying to do? I don't understand the multiplication in there. If you wanted to store the whole 32bit number as an integer you would call move() and shift the value over.

**Legolas** · 31-05-2007, 01:45 PM

I need to store the floating point value in a 32 bit integer, where the 1st bit is the sign, bits from 2 to 20 are holding the integer part and 21 to 32 the fraction part. It's for 3d on Nintendo DS, because this particular format number is used to handle matrix transformations via hardware.

**Robert Kosek** · 31-05-2007, 01:47 PM

Ouch. I really don't know how to help you then. Maybe someone else can who has a better knowledge of those things.

**savage** · 31-05-2007, 01:54 PM

Do you have an example of type of input and expected output?

**Legolas** · 31-05-2007, 02:09 PM

Originally Posted by savage

Do you have an example of type of input and expected output?

Thanks, mates. At this time I'm at work. As soon as I'll come back home I'll make it

**savage** · 31-05-2007, 02:10 PM

btw, in Delphi 6 the previous code you posted does not compile as I get an invalid typecast error.

The following code appears to convert between the floats and Ints ( to a few decimal places ), but I'm not sure if it is what you are looking for...

[pascal]
function FloatToInt32(n: single): integer;
begin
result := Round((n) * (1 shl 12));
end;

function Int32ToFloat(n: integer): single;
begin
result := ( (n) / (1 shl 12) );
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
ShowMessage( FloatToStr( Int32ToFloat( FloatToInt32( 1.1 ) ) ) );
end;
[/pascal]

**Legolas** · 31-05-2007, 07:16 PM

I have tried every possible combination of trunc/round/shl/shr, but the obvious one:

[pascal]function floattof32(n: cfloat): cint32; inline;
begin
floattof32 := trunc((n) * (1 shl 12));
end;[/pascal]

This one seems working as expected, though I have learned "never think that something works, even when it seems working". Thanks again, guys! And stay tuned, because I'm making some other strange type conversion routines :lol: